This invention pertains to a speed responsive shutoff mechanism for a pneumatic nut running tool wherein the motor exhaust flow is interrupted at a predetermined minimum speed of the tool motor as the motor approaches the stall condition.
In the art of control devices for pneumatic power tools, in particular nutsetters, screwdrivers and the like, there have been a number of inventions dealing with a speed responsive valve for shutting off the motor inlet air flow when the motor speed approaches the stall condition. U.S. Pat. Nos. 3,904,305; 4,004,859 and 4,120,604 disclose pneumatic tools which include speed responsive valve mechanisms for shutting off fluid flow to the motor inlet port when the motor approaches the stall condition from a relatively high speed operating condition. U.S. Pat. Nos. 3,785,442; 3,791,458 and 3,850,553 disclose speed responsive devices which generate a pressure fluid signal at a predetermined minimum speed of the tool motor to cause a servo-operated valve to shut off motive fluid flow.
A common disadvantage of the devices disclosed in the first three patents abovementioned is that although the speed responsive devices operate to shut off directly the fluid flow to the motor inlet port, these devices all require a so-called bypass valve for providing motive air to start the motor so that the respective centrifugal force responsive mechanisms can open a motor inlet fluid passage which is then reclosed on decreasing motor speed.
The three last mentioned patents also disclose somewhat complicated mechanisms which require speed responsive valve devices that provide a pressure fluid signal for operating separate motive fluid shutoff valves.